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I need a reliable low battery early warning system.

I'm using Epson RX8900SA RTC and CR2032 battery. I want to measure battery voltage (probably every 24 hour or more).

How can I do this without draining battery too much?

My first idea is to use low cost J-FET input amplifier as buffer (voltage follower circuit) and feed microcontroller ADC from its output. If this is good idea - which op-amp can I use? TL082 has too high supply requirements for me (I have 3.3V and 5V supplies avaliable, and TL082 needs at least 6V to do anything). Or maybe I can just use regular low cost single supply op-amp like LM358?

I also want to use some mosfet to connect resistor as load (around 10-20kOhm) at the moment of measurement to make measurements more reliable. Maybe I could just connect 10k-20k resistor directly to ADC and connect that (on high side) by mosfet when I take measurement?

My question is totally diffrent from this.

I'm not asking about ohms law. I asked for solution or some improvement/comment for my idea with high impedance measurement with op-amp or mosfet switch.

Come on guys...

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I also want to use some mosfet to connect resistor as load (around 10-20kOhm) at the moment of measurement to make measurements more reliable. Maybe I could just connect 10k-20k resistor directly to ADC and connect that (on high side) by mosfet when I take measurement?

I question the need for the 10-20 kOhm resistor. But the high side switch between ADC and battery should be the only thing you need. I don't see why you need a voltage buffer for a battery. When the FET is off, leakage through the FET should be minimal, so minimal draw on the CR2032. With no buffer, there is also no draw on VCC. What is the VREF for the ADC? Is it in all cases, higher than the voltage of the CR2032? The ADC cannot measure any voltage higher than VREF.

schematic

simulate this circuit – Schematic created using CircuitLab

For M2, a BSS84 may be a good choice (but double check everything).

For M1, a BSS138 is probably the best choice (but double-check).

For both transistors, there are many parts available that will work.

It would probably be a good idea to put a small capacitor on the ADC input to help with noise filtering. It might not be needed, in which case you can leave the component off. But if you need it, and the footprint is not on your PCB, then you have no options.

The cap, if used, will steal a tiny bit of charge from the battery every time you turn on M2, but that should be negligible.

I chose R2 and R1 somewhat randomly. You can probably use different values if you want. Anywhere from like 1k to around 1M will probably work.

R3 is probably not needed, but people frequently recommend a series resistor for the gate drive of any FET, even these small low-power FET's. So I put it in there just to avoid their comments.

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  • \$\begingroup\$ Will switch like this work properly when microcontroller (and gate voltage) is off? How to choose transistor? Any logic level P-channel mosfet will do the job? \$\endgroup\$
    – Kamil
    May 12, 2021 at 22:31
  • \$\begingroup\$ No, you should use two transistors if you want to make it be off when microcontroller is off. PMOS swich, with gate controlled by NMOS. NMOS gate controlled by microcontroller. \$\endgroup\$
    – user57037
    May 13, 2021 at 2:11
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    \$\begingroup\$ I think I got this. Little schematic of that switch (with specyfic types of transistors) would help me a lot, but if I'm asking for too much I will figure it out myself on some breadboard. Thanks and have a nice day :) \$\endgroup\$
    – Kamil
    May 14, 2021 at 21:12
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    \$\begingroup\$ I added a schematic. \$\endgroup\$
    – user57037
    May 14, 2021 at 21:44

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